My invention relates to the correction of the distorting effects of limited bandwidth transmission media on modulated data signals.
The principal impediment to accurate reception of high-speed data signals transmitted over limited bandwidth, e.g., switched voiceband telephone, transmission channels is that form of distortion known as intersymbol interference. This phenomenon is a manifestation of the fact that a pulse passing through a band-limited channel expands in the time domain. As a result, each sample of the received signal is not simply derived from a single transmitted data symbol but, rather, some combination of symbols. Other impairments include phase jitter and additive noise.
Linear intersymbol interference, in particular, is manifested in that each sample of the received signal contains a linear combination of a transmitted symbol--which the sample nominally represents--with symbols which precede and succeed it in the data stream. Among known techniques which compensate for the distorting effects of linear intersymbol interference in both baseband and passband, e.g., quadrature amplitude modulated (QAM), signals are linear feedforward equalization and linear decision feedback equalization. In accordance with the former technique, each sample of the received signal is weighted with a linear sum of past and future samples prior to a decision being made as to the value of the transmitted symbol. In accordance with the latter technique, a weighted linear sum of past decisions is combined with each sample, again prior to a decision being made as to the value of the transmitted symbol. See, for example, my U.S. Pat. No. 3,974,449 issued Aug. 10, 1976.
Nonlinear intersymbol interference is manifested in that each sample of the received signal includes a linear combination of products of the current, past and future modulated data symbols, and/or (in the case of QAM, for example) the complex conjugates of such data symbols. In transmission systems that employ linear modulation, such as QAM, the effect is to reduce the margin against noise. Indeed, for data rates above 4800 bps, nonlinear distortion is the dominant impairment on many voiceband channels. At least one arrangement is known which compensates for nonlinear intersymbol interference in baseband data signals. See, e.g., U.S. Pat. No. 3,600,681 issued Aug. 17, 1971 to T. Arbuckle. However, the known arrangements will not, in general, effectively compensate for nonlinear intersymbol interference in modulated data signals.